add hasher<string_view>

Summary: Add a specialization of `folly::hasher<std::string_view>` parallel to the specialilzation of `folly::hasher<std::string>`.

Reviewed By: luciang

Differential Revision: D28921343

fbshipit-source-id: 77213d9ff66cec57b6d36b214c41e479e1d9455d

folly/hash/Hash.h

@@ -33,6 +33,10 @@
 #include <folly/hash/SpookyHashV2.h>
 #include <folly/lang/Bits.h>
 
+#if FOLLY_HAS_STRING_VIEW
+#include <string_view>
+#endif
+
 namespace folly {
 namespace hash {
 
@@ -561,6 +565,20 @@ struct hasher<std::string> {
 template <typename K>
 struct IsAvalanchingHasher<hasher<std::string>, K> : std::true_type {};
 
+#if FOLLY_HAS_STRING_VIEW
+template <>
+struct hasher<std::string_view> {
+  using folly_is_avalanching = std::true_type;
+
+  size_t operator()(const std::string_view& key) const {
+    return static_cast<size_t>(
+        hash::SpookyHashV2::Hash64(key.data(), key.size(), 0));
+  }
+};
+template <typename K>
+struct IsAvalanchingHasher<hasher<std::string_view>, K> : std::true_type {};
+#endif
+
 template <typename T>
 struct hasher<T, std::enable_if_t<std::is_enum<T>::value>> {
   size_t operator()(T key) const noexcept { return Hash()(to_underlying(key)); }

folly/hash/test/HashTest.cpp

@@ -628,6 +628,12 @@ TEST(Hash, Strings) {
   EXPECT_EQ(h2(a2), h2(a2.str()));
   EXPECT_EQ(h2(a3), h2(a3.str()));
   EXPECT_EQ(h2(a4), h2(a4.str()));
+
+  // Check compatibility with std::string_view.
+  EXPECT_EQ(h2(a1), h2(std::string_view{a1}));
+  EXPECT_EQ(h2(a2), h2(std::string_view{a2}));
+  EXPECT_EQ(h2(a3), h2(std::string_view{a3}));
+  EXPECT_EQ(h2(a4), h2(std::string_view{a4}));
 }
 
 namespace {